Hydrogen peroxide (H2O2) is a well-known chemical having strong oxidizing properties and is usually sold in aqueous solution. Aqueous hydrogen peroxide is available in a wide range of concentrations and has a variety of commercial applications, as a disinfectant, antiseptic, bleaching agent, oxidizer (including in chemical reactions), and as a propellant (e.g., in rocketry). A noteworthy characteristic of hydrogen peroxide is that its decomposition byproducts are innocuous.
Hydrogen peroxide may undergo decomposition either in the vapor phase or condensed phase, e.g., in aqueous solution, resulting in decomposition products of oxygen gas and water. The overall decomposition reaction is as follows:H2O2→H2O+½O2↑  (1)
One developing end-use application of hydrogen peroxide is in the field of air pollution control, in the treatment and removal of contaminants present in flue gas streams from stationary combustion sources, e.g. electric utility power plants that utilize fossil fuels.
Combustion of fuels such as coal, coke, natural gas or oil typically results in the presence of pollutants in the combustion flue gas stream resulting from the combustion process or derived from impurities present in the fuel source. Electric utility power plants that burn coal are a significant source of such combustion process air pollutants, but other stationary fuel-burning facilities such as industrial boilers, waste incinerators, and manufacturing plants are also pollution sources.
The primary air pollutants formed by these stationary high temperature combustion sources are sulfur oxides (e.g., SO2 and SO3), also called SOX gases, and nitrogen oxides, also called NOX gases, both of which are acid gases. Other combustion pollutants of concern in these combustion flue gases include other acid gases such as HCl and HF, Hg (mercury), CO2 and particulates. In addition, residual amounts of unreacted ammonia (NH3), used in the treatment of flue gas NOX in selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) systems, is another contaminant of concern in combustion flue gas streams. These individual pollutant components from stationary combustion sources have been subject to increasingly more stringent regulatory requirements over the past decades, and emission standards are likely to be tightened in the future.
Aqueous hydrogen peroxide has been proposed for various applications in the treatment of combustion flue gas streams for removal of contaminants. However, there is still a need for air pollution control treatment procedures that utilize hydrogen peroxide in a highly efficient manner.
The present invention provides an air pollution control method for the effective control of flue gas stream contaminants, particularly NOX and Hg and residual ammonia (in SCR-treated or SNCR-treated combustion flue gas streams), utilizing activated hydrogen peroxide that is introduced as an oxidizing reactant into the flue gas stream. The novel hydrogen peroxide activation system of this invention is not disclosed or suggested in prior art treatments for abating SOX, NOX and other gaseous contaminants in combustion flue gas streams.
U.S. Pat. No. 4,213,944 of Azuhata et al. (Hitachi) discloses a process for removing nitrogen oxides from a hot gas stream containing the same by adding a reducing agent, preferably ammonia, and hydrogen peroxide into hot gas stream at an elevated temperature of 400° C.-1200° C. to decompose the nitrogen oxides to nitrogen gas and water. The hydrogen peroxide is added concurrently with the ammonia and is said to increase the activity of the ammonia, particularly at gas temperatures of 400° C.-800° C., by decomposing the ammonia to make it reactive with the NOX. Sufficient hydrogen peroxide is added with the ammonia so that excess unreacted ammonia is also decomposed.
U.S. Pat. Nos. 5,120,508 and 4,783,325 of Jones (Noell) disclose methods of converting NO to NO2 in a flue gas stream by injecting a gas containing a peroxyl initiator and oxygen into the NO-containing gas stream. The peroxyl initiator is preferably propane but may also be other hydrocarbons or hydrogen peroxide or hydrogen. The resultant NO2-containing gas stream is then treated in an absorption section to remove NOX and SOX with a dry sorbent such as nahcolite or trona, the dry sorbent being captured in a baghouse before the treated gas stream is discharged into the atmosphere.
U.S. Pat. No. 5,670,122 of Zamansky et al. (Energy & Environmental Research) discloses a method for removing NO, SO3, CO, light hydrocarbons and mercury vapor (Hg) from combustion flue gas by injecting into the gas stream atomized droplets of either hydrogen peroxide or a mixture of hydrogen peroxide and methanol, to convert the respective gas contaminants to NO2, SO2, CO2 (for the CO and light hydrocarbons) and HgO. The treatment is carried out at a gas temperature of about 377° C. to about 827° C., and the reaction products are subsequently removed in a downstream scrubbing operation. The treatment also may be carried out in combination with SNCR NOX reduction technology, with the SNCR-treated combustion gas stream being treated downstream with the H2O2 or H2O2/CH3OH injection treatment.
U.S. Pat. No. 6,645,450 of Stoltz et al. (Steen Research) discloses a method of controlling odors and noxious components, e.g., in effluent gas streams from food processing plants, by treating the gaseous effluent stream in a wet scrubber system with aqueous hydrogen peroxide and an additive, preferably aqueous ferrous sulfate solution, that serves to catalyze the rapid decomposition of hydrogen peroxide into hydroxyl radicals.
U.S. Pat. No. 6,676,912 of Cooper et al. (NASA) discloses a method of removing NO from stationary combustion gas streams by injection of H2O2 into the gas stream to oxidize NO to NO2 and HNO3 and HNO2, which species are more readily recovered via aqueous wet scrubbing. The nitrogen acids and residual NO2 are then removed via wet scrubbing with water or an aqueous alkaline medium or via passage of the flue gas stream through a particulate alkaline sorbent in a baghouse. The method may optionally include a preliminary flue gas desulfurization scrubbing step to remove SO2, prior to the H2O2 injection.
U.S. Pat. No. 6,793,903 of Parrish et al. (NASA) and U.S. Pat. No. 6,955,799 of Parrish et al. (NASA) disclose methods of oxidizing nitric oxide (NO) into nitrogen dioxide (NO2) by the high temperature decomposition of hydrogen peroxide into hydroxyl (HO.) and hydroperoxyl (HOO.) oxidative free radicals. A hydrogen peroxide solution is impinged onto a heated surface in a stream of nitric oxide, where the hydrogen peroxide decomposes to produce the oxidative free radicals. The heated surface is preferably coated with a catalytic material, e.g., Fe(II or III), Cu (II), Cr(II), Pt black, Ag, Pd (col. 3, lines 27-52).
In the method of Parrish et al. '799, the heated surface may either be coated with a catalytic material or may contain a solution or dispersion of a catalytic or reactive material. In the latter embodiment, the hydrogen peroxide is added to an aqueous solution or dispersion containing a salt or metal oxide that decomposes hydrogen peroxide to produce water and oxygen (col. 4, lines 33-60). The resultant oxygen that results from the decomposition of hydrogen peroxide has a low solubility in water and is released from the solution/dispersion into the nitric oxide stream to oxidize the NO to NO2 (col. 4, lines 33-60 & col. 5, lines 16-36).
U.S. Statutory Invention Disclosure No. H1948H of Rusek et al. (U.S. Navy) discloses a method, applicable to hydrogen peroxide-fueled rocket thrusters, of decomposing hydrogen peroxide that is flowed over a fixed bed catalyst containing a H2O2— catalytically-active compound containing a transition metal cation mixed with an alkaline promoter. A preferred catalyst is tetravalent manganese with Na+ or K+ ions as the alkaline promoter, the catalyst being calcined and carried on an inorganic polar substrate.
U.S. Patent Publication No. 2004/0197252 of Parrish et al. (NASA) discloses the conversion of nitric oxide (NO) in a gas stream into nitrogen dioxide (NO2) using concentrated hydrogen peroxide that is fed as a monopropellant into the gas stream via a catalyzed (rocket) thruster assembly. The catalyst, preferably a mixed catalyst of molybdenum oxides and manganese oxides on a catalyst support mounted in the thruster nozzle, decomposes the hydrogen peroxide into hydroxyl ions (OH−) and/or hydroperoxy ions (OOH−) which react with the nitric oxide in the gas stream.
U.S. Patent Publication No. 2008/0213148 of Parrish et al. (NASA) discloses a method of reducing NOX emissions from flue gas streams, using a gaseous chlorine dioxide treatment step and at least one aqueous hydrogen peroxide scrubbing solution treatment step. In this invention, the chlorine dioxide treatment step serves primarily to oxidize NO to NO2.
U.S. Patent Publication No. 2008/0241030 of Parrish et al. (NASA) discloses a method of reducing emissions from flue gas streams, using multiple aqueous hydrogen peroxide scrubbing treatment steps and an intermediate gaseous chlorine dioxide treatment step, to treat NOX—, SOX— and heavy metal-containing flue gas streams. The gaseous chlorine dioxide treatment is used to remove heavy metals such as mercury from the flue gas stream, as well as any NO that is not previously oxidized by the first aqueous hydrogen peroxide scrubbing step.
Chlorine dioxide, mentioned in the above-noted two Parrish et al. patent publications, is a strong oxidizing agent generally used in water treatment and pulp bleaching. Hydrogen peroxide has been used in the preparation of chlorine dioxide, as described in the following two patent references.
U.S. Pat. No. 2,332,181 of Soule (Mathieson Alkali Works) discloses that chlorine dioxide (ClO2) may be formed by the reaction in an acidic medium of a metal chlorate, e.g., sodium chlorate, and hydrogen peroxide, the latter functioning as a reducing agent.
U.S. Patent Publication No. 2003/0031621 of Gravitt et al. discloses an improvement in the production of chlorine dioxide, in which hydrogen peroxide and aqueous alkali metal chlorate, in the presence of a mineral acid, are sprayed into a spherical reaction chamber to form a foam that promotes the efficient production of chlorine dioxide. The chlorine dioxide is recovered from the reaction apparatus, e.g. in a stripper column.
The present invention provides a highly efficient means for activating hydrogen peroxide, particularly for its reaction with contaminants present in combustion flue gas streams. The invention is also useful for the catalytic activation or reaction of other reactive compounds, as described in the specification below.